104 SUMMARY OF CURRENT RESEARCHES RELATING TO 



horst and Borscow,* advocate another hypothesis. According to their 

 view the chief cause of all movements of Bacillarice is the energy of 

 the osmotic processes. The proofs which they adduce are likewise 

 indirect, and consist essentially of the following : — 



(1) In the first place, no one has seen in the Diatomacese either 

 pseudopodia, or in general any external protoplasm. The supposed 

 extreme fineness of the former is not a valid reason for not seeing 

 them, as the very fine pseudopodia of some Protozoa are plainly seen 

 without the use of reagents. The invisibility of the external proto- 

 plasm is also the more extraordinary, as it has been sought for by 

 such practised microscopists as Max Schultze, Cohn,| Pfitzer, and 

 Engelmann.J 



The (in the opinion of the author) most doubtful existence of an 

 external protoplasmic layer in the Oscillarise, were it even actually 

 confirmed, affords no solid grounds for assuming such a layer to be 

 present in the Diatomaceae. 



(2) Another objection may be urged against the protoplasm 

 theory. The character of the movements does not at all suggest 

 those of such organisms as make use of either the contractility of the 

 protoplasm en masse, or that of the pseudopodia. In the diatoms 

 we do not see the uniformity and sluggishness characteristic of the 

 former, but on the contrary, the movement not unfrequently takes a 

 backward direction, so that the alga sometimes goes back suddenly 

 a comparatively great distance. 



As an illustration, the author adduces the description of this 

 phenomenon given by Borscow. § " The cell, until then round," he 

 says, " suddenly makes an energetic rectilinear forward movement, 

 stops for a time, and after this short pause makes a rapid movement 

 straight as an arrow in the opposite direction, whereupon a pause 

 again ensues." In other cases we may " observe that several forward 

 movements of the cell, broken by short pauses, follow one another ; 

 then ensues a longer state of rest, and subsequently a retrograde 

 movement takes place in the same manner, i. e. hy jerks" 



Nothing similar to this is found in the Amoehce, and other Ehizopoda, 

 or in the plasmodia of Myxomycetes, on their changing their locality. 



Eegard must also be had to the fact that a diatom in moving 

 is frequently not in a horizontal position, but keeps one of its ends 

 directed somewhat upwards, so that the supporting surface is in con- 

 tact with its other end only, consequently with a very inconsiderable 

 portion. It is difficult to conceive how so small a quantity of proto- 

 plasm which touches the surface in such a case could put in motion 

 by its contraction not only the whole (sometimes large) cell, but also 

 a considerable number of foreign bodies, such as grains of sand, &c., 

 which often cling to the alga.|| 



* ' Die Siisswasser-Bacillarien des siidwestlichen Russlands,' 1873, p. 34. 



t "Beitrage zur Physiologie der Phycochromaceen und Florideeu," Arch. 

 Mikr. Anat., iii. p. 50. 



J Bot. Ztg., loc. cit., p. 55. 



§ Loc. cit., p. 35. 



II This elevation of one end of the alga is easily explained by assuming that 

 the specific gravity of it fluctuates about 1, and in favour of this the fact may be 



